Superconducting pi qubit with a ferromagnetic Josephson junction

Solid-state qubits have the potential for the large-scale integration and for the flexibility of layout for quantum computing. However, their short decoherence time due to the coupling to the environment remains an important problem to be overcome. We propose a new superconducting qubit which incorporates a spin-electronic device: the qubit consists of a superconducting ring with a ferromagnetic pi junction which has a metallic contact and a normal Josephson junction with an insulating barrier. Thus, a quantum coherent two-level state is formed without an external magnetic field. This feature and the simple structure of the qubit make it possible to reduce its size leading to a long decoherence time.Comment: 4 pages, 3 figure

Classical Coulomb three-body problem in collinear eZe configuration

Classical dynamics of two-electron atom and ions H$^{-}$, He, Li$^{+}$, Be$^{2+}$,... in collinear eZe configuration is investigated. It is revealed that the mass ratio $\xi$ between necleus and electron plays an important role for dynamical behaviour of these systems. With the aid of analytical tool and numeircal computation, it is shown that thanks to large mass ratio $\xi$, classical dynamics of these systems is fully chaotic, probably hyperbolic. Experimental manifestation of this finding is also proposed.Comment: Largely rewritten. 21 pages. All figures are available in http://ace.phys.h.kyoto-u.ac.jp/~sano/3-body/index.htm

Compact Binary Formation in Open Star Clulsters I: High Formation Efficiency of Gaia BHs and Their Multiplicities

Gaia BHs, black hole (BH) binaries discovered from database of an astrometric telescope Gaia, pose a question to the standard binary evolution model. We have assessed if Gaia BHs can be formed through dynamical capture in open clusters rather than through isolated binary evolution. We have performed gravitational $N$-body simulations of $100$ open clusters with $10^5 M_\odot$ in total for each metallicity $Z=0.02$, $0.01$, and $0.005$. We have discovered one Gaia BH-like binary escaping from an open cluster, and found that the formation efficiency of Gaia BHs in open clusters ($\sim 10^{-5} M_\odot^{-1}$) is larger than in isolated binaries ($\sim 10^{-8} M_\odot^{-1}$) by 3 orders of magnitude. The Gaia BH-like binary is the inner binary of a triple star system. Gaia BHs can have tertiary stars frequently, if they are formed in open clusters. We have estimated the number of Gaia BHs in the Milky Way disk to $1.6 \times 10^4$, large enough for the number of Gaia BHs discovered so far. Our results indicate that the discoveries of Gaia BHs do not request the reconstruction of the standard binary evolution model, and that Gaia BHs are a probe for the dynamics of open clusters already evaporated.Comment: 7 pages, 6 figures, 1 table, submitted to MNRA